This invention relates to phase-sensitive frequency detector circuits of an improved type which minimize the tendency of the tuned circuits to ring after a driving signal is applied or removed, whereby the circuit is particularly effective for the detection of pulses or transients, as well as long term signals which may appear in the presence of other spurious frequencies.
It is frequently desirable to detect the presence of a signal of predetermined frequency, which may also appear in the presence of undesired signals either of long term or short term duration. It is also desirable at times to provide information on the frequency spectrum of a signal by determining the presence of particular frequencies in real time, and to indicate such presence as a binary output which is compatible for use with a digital computer. More particularly, it is desirable to provide binary-indicated detection which is delivered during the signal and which ends immediately after the detected signal is terminated.
It is well known that when a wide band signal is passed through a tuned circuit having a given resonant or cross-over frequency, the phase of the output signal whose frequencies are above cross-over is shifted about 180.degree.. If two paths of tuned circuits, low-pass, high-pass, or band-pass are coupled to receive an input signal, and if their pass-bands overlap somewhat, then the two output signals resulting from an input signal which is either entirely above both cross-overs or entirely below both cross-overs will be essentially in phase with each other. On the other hand, in the case of an input signal lying between the two cross-overs, the output signal from one filter will be about 180.degree. out of phase with the output signal from the other filter. This difference in phase can be used to drive a concidence detector responsive to their relative phases so as to determine when the dominant component of an input signal applied to the two filters results in in-phase outputs from the filters, or out-of-phase outputs therefrom. Those frequency components which are outside the pass bands will produce a different response in the coincidence detector than those which are between the pass bands.
My U.S. Pat. No. 4,215,280 shows such a detector system for detecting the presence of selected frequency components of an input signal, and operative to deliver an output responsive thereto when an input signal contains frequency components within certain limits determined by the design of the filter system itself. This filter system generally comprises two filter paths in which dominant input frequencies which are outside of both filter pass bands are found to produce components which have in-phase relationship at the outputs of the filters, but wherein signals located between the cross-over frequencies of the two paths are out-of-phase with respect to each other at the outputs of the paths, so that a coincidence detector can be made to deliver an appropriate binary output signal resulting from detection of these mutual phase relationships. This patent shows various systems using low pass filters, high-pass filters, combinations thereof, or band pass filters in each path, whose outputs are coupled to the coincidence detector. The present invention uses detectors of this type to accomplish its purpose, which is to provide an improved detection system which minimizes ringing effects of the tuned circuits.
Whenever a signal is applied to a filter, or whenever the signal is abruptly removed from the filter, the resonant circuit in the filter system tends to ring at its own natural frequency. This is particularly true of transient signals driving filter systems. The steep leading edge of a transient signal is ideal in shape to excite a tuned circuit to ring at its own natural frequency. Likewise, cessation of the transient provides another steep edge exciting more ringing of the tuned circuit. Since the present system uses binary gates as detectors, such ringing effects are very damaging, since they falsely indicate the passage of signals within the pass band. Damping of the circuit will, of course, eventually attenuate the ringing effect, but before attenuation can occur the output of a detector is severely obscured by spurious signals, for instance, during analysis of transient signal components, etc., i.e. pulse signals.
In the analogic case, a very similar situation occurs. If analogic signals are suddenly turned on or off, without smoothing, severe ringing will occur leading to spurious output. Generally, in the analogic case smoothing transients are in fact present which tend to reduce the amplitude of ringing oscillations, but in the digital case smoothing transients are absent and therefore such circuits must be considered on the whole as noisier.
Ringing of the tune circuits, can be analyzed in the following manner.
When the signal is first applied to a two path filter, both of its resonant circuits will be excited by the same leading edge and will ring initially in phase. A similar effect also occurs immediately after a signal is abruptly cut off. The duration of the ringing, of course, depends upon the circuit damping, but the beginning of the ringing in two similar tuned circuits will be in phase only initially and will almost immediately begin to go out of phase, assuming that the tuned circuits are not tuned to identical frequencies, whereby each tuned circuit resonates at a slightly different frequency from the other. As a result, what can be described as a "beating phenomenon" occurs since the oscillations in the two circuits will appear to drift alternately out-of-phase and then back into phase again at a regular rate. Thus, at the outset when two similar tuned circuits are driven, two effects will be at work to prevent output from a coincidence detector coupled thereto. One is the effect of resistive damping in both filters, and the other is the result of their tendency to immediately drift out-of-phase as viewed at the coincidence detector. Ordinarily, sufficient damping is designed into each circuit so that after a few cycles of ringing the output will fall below the threshold signal level to which the coincidence detector is responsive. The present system takes advantage of the tendency of the differently tuned circuits to drift out-of-phase, in order to eliminate output from the coincidence detector due to such ringing. This is done by phase comparison of the binary response signals from the coincidence detector. The following discussion will be illustrated in terms of filter systems of the types shown in my above-mentioned U.S. Pat. No. 4,215,280, employing one or more filter circuits in multiple paths leading to coincidence detectors.